Revision anterior cruciate ligament (ACL) reconstruction represents an infrequent but clinically important challenge in orthopaedic practice. Technical issues require specific revision techniques to address complications such as retained hardware, bone tunnel defects, and incorrect tunnel placement. Moreover, it is commonly reported that the results of revision surgery remain inferior to primary reconstructions. These poorer outcomes include worse patient based outcomes, increased knee laxity, higher graft failure rate, meniscal degeneration, and chondral lesions. A number of reasons for the poorer outcome rate have been proposed, including compromised tunnel location, pathology untreated during the primary reconstruction, and greater reliance on allografts for revisions. In order to evaluate the contributions of these and other factors to outcome, large numbers of patients who are undergoing revisions must be identified and followed prospectively. However, due to the relative infrequency of revision ACL reconstructions in any one center, a large multi-center study is necessary to accumulate enough subjects over a reasonable time frame to allow for multivariable analyses. With this in mind, the Multi-center ACL Revision Study (MARS) group was established as an 87 surgeon multi-center group to perform a prospective longitudinal cohort analysis of revision ACL reconstruction. This is a mixed group of academic and private practice physicians and has been supported and endorsed by the American Orthopedic Society for Sports Medicine (AOSSM). The group has been enrolling revision ACL reconstruction patients for over three years and has accumulated greater than 800 patients. This grant uses a prospective cohort study design and focuses on the modifiable predictors for ACL revision outcome at two-year follow-up. The overall goal is to elucidate the factors responsible for success or failure of ACL revision. We hypothesize that modifiable independent variables exist at the time of revision ACL reconstruction that will predict revision ACL reconstruction outcomes. This will be accomplished by focusing on four Specific Aims. Specific Aim 1 will determine if graft choice predicts incidence of graft rerupture, reoperation, return to sports function and activity level 2 years post-surgery. Specific Aim 2 will determine if modifiable surgical factors influence graft rerupture rates and return to sports function. Specific Aim 3 will determine if both prevalence and/or degree of meniscal and chondral damage measured at the time of the revision ACL reconstruction predicts activity level, return to sports function, and pain and swelling symptoms 2 years post-surgery. Specific Aim 4 is an exploratory aim, which determines if various rehabilitation factors influence the incidence of graft rerupture or return to sports activity 2 years following revision ACL surgery. The results of this study will drive the field of Orthopaedics Sports Medicine, in that it will guide practitioners to most appropriately choose the graft for reconstruction, modify surgical approaches and techniques, and manage meniscal and chondral injuries in a way to provide optimal outcome.